1. Field of the Invention
This invention relates to addressing liquid crystal displays (LCDs) to provide a plurality of gray shades or levels for the displayed image and more particularly to an apparatus and method for providing a very high number of gray levels for a fast-responding passive matrix LCD.
LCDs are becoming increasingly useful for displaying images not only in projection systems but as screens for television receivers and computers. As a consequence, there is a demand for even faster-responding, high information content LCDs that can provide a very large number of gray levels between white and black or a large color palette.
2. Discussion of the Prior Art
One method of providing gray scale for an LCD is known as frame modulation, exemplified by U.S. Pat. Nos. 4,752,744 and 5,062,001 and an article by Y. Suzuki, et al., "A Liquid-Crystal Image Display," 1983 SID Digest of Technical Papers XIV 32-33 (1983). In these frame modulated systems the pixels forming the image on the screen are turned "on" and "off" in different frames correlated to the gray level or shade of color desired. When applied to faster-responding LCDs, however, frame modulation causes "flicker" and "swim". The former is perceived by the viewer as if the image were being rapidly turned on and off, and in the latter, the image appears to have ripples or waves passing through it.
The so-called "pulse-width modulation" gray scale system, exemplified by U.S. Pat. No. 4,427,978 issued Jan. 24, 1984 and an article by H. Kawakami, et al., "Brightness Uniformity in Liquid Crystal Displays," 1980 SID Digest of Technical Papers XXI, 28-29 (1980), is limited in the number of distinct gray levels that it can produce. Pulse width modulation is physically incapable of providing gray levels of the order of 256 which are desirable to bring out the fine detail of images required in "multimedia" applications of LCDs. In pulse-width modulated systems, the pulses become narrower and the high frequency content of the drive signals increases with the number of gray levels. These higher frequencies are cut off by the low-pass RC filter action of the LCD panel, which makes it difficult to realize more than about 4 to 7 gray levels on the display.